Arrangement in a pressing tool for sheet metal forming

ABSTRACT

Arrangement in a pressing tool for sheet metal forming in order to shut off or retard an oil flow in a hydraulic system at an arbitrarily chosen time in coordination with the closing and opening movements of the said tool. When the tool closes, a non-return valve (6) permits a flow in one direction from a primary hydraulic cylinder/piston unit (7), which is thereby compressed, to an accumulator (8). A first valve (9) selectively permits a flow between the accumulator (8) and a secondary hydraulic cylinder/piston unit (10) which, in the event of such a flow, passes from a compressed basic position to an expanded position. Parallel with the non-return valve (6) is a second valve (11) which, when the primary hydraulic cylinder/piston unit (7) has been relieved by opening of the tool, will selectively permit a return flow to the primary hydraulic cylinder/piston unit (7) from the accumulator (8) or, if the first valve (9) is also opened, from the secondary hydraulic cylinder/piston unit (10), which is designed to be then returned to the compressed basic position.

The present invention relates to an arrangement in a pressing tool forsheet metal forming in order to shut off or retard an oil flow in ahydraulic system in coordination with the closing and opening movementsof the said tool.

In sheet metal forming operations, for example, it is usual to arrange apassive hydraulic system in the pressing tool. This is done in order tobe able to shift some of the pressing force to another position andanother direction, for example in order to be able to perform yetanother operation in the same tool. Such a system can be achieved byarranging a primary hydraulic cylinder/piston unit in a lower tool halfin such a way that the piston is pressed into the cylinder when the toolis closed. The oil flow from the primary hydraulic cylinder/piston unitcan thus be used in order to produce a stroke in a secondary hydrauliccylinder/piston unit. The timing of the said stroke coincides with theclosing of the tool.

In certain cases it is desirable to be able to freely determine thepoint in time at which the secondary force/movement is to act. Where apassive system of the above-mentioned type is used it is thereforedesirable to be able to retard the oil flow between the primary and thesecondary hydraulic cylinder/piston unit. Such a retardation is alreadyknown. This retardation is achieved by allowing a primary hydrauliccylinder/piston unit arranged in a lower tool half of a pressing tool toact in opposition to a gas-filled spring arranged in an upper tool half.A valve is arranged and designed to selectively permit a flow betweenthe primary and the secondary hydraulic cylinder/piston unit. By keepingthe valve closed when the tool is closing, the gas-filled spring in theupper part of the sheet metal forming tool is compressed against thepiston rod of the hydraulic cylinder/piston unit. The said piston rod isnot pressed into the cylinder when the tool is being closed, but onlywhen the valve is opened after predetermined delay. The gas-filledspring then presses the piston rod into the cylinder so that the oilflows from the latter to the secondary hydraulic cylinder/piston unit,which can then supply force and movement. One limitation to the solutiondescribed is that the pressing tool must be in the fully closed positionin order to be able to supply a satisfactory oil flow. This means thatthe time at which the secondary force/movement will act cannot bedetermined with complete freedom.

The object of the present invention is to produce an arrangement whichcan shut off or retard an oil flow in a hydraulic system in a pressingtool for sheet metal forming for an arbitrarily chosen period. Thisobject is achieved according to the invention in that it comprises: anon-return return valve designed, when the tool is being closed, topermit a flow in one direction from a primary hydraulic cylinder/pistonunit, which is thereby compressed, to a pressurised accumulator; a firstvalve designed to selectively permit a flow between the accumulator anda secondary hydraulic cylinder/piston unit, which is designed as aconsequence of such a flow to move from a compressed basic position toan expanded position; and parallel with the non-return valve a secondvalve, designed so that when the primary hydraulic cylinder/piston unithas been relieved by opening of the tool, it will selectively permit areturn flow to the primary hydraulic cylinder/piston unit from theaccumulator or, if the first valve is also made to open, from thesecondary hydraulic cylinder/piston unit, which is designed to be thenreturned to the compressed basic position, and wherein the accumulatoris of dimensions sufficient to accommodate the oil which is in thesystem when both the hydraulic cylinder/piston units are compressed.

According to a special characteristic of the invention, the pressure inthe accumulator is always equal to or greater than the working pressurein the hydraulic system.

According to a further special characteristic of the invention, a pistonwhich separates the oil from pressure generating means is located in theaccumulator.

According to a further special characteristic of the invention, gas isused as the pressure generating means.

According to a further special characteristic of the invention, a coilspring is used as pressure generating means.

According to a further special characteristic of the invention, thesecondary hydraulic cylinder/piston unit is designed to be returned tothe compressed basic position by means of a gas-filled spring.

According to a further special characteristic of the invention, thesecondary hydraulic cylinder/piston unit is designed to be returned tothe compressed basic position by means of a coil spring.

According to a further special characteristic of the invention, thesecondary hydraulic cylinder/piston unit is designed to be returned tothe compressed basic position by means of a gas under pressure.

According to a final special characteristic of the invention, the pistonin the accumulator is designed to rest against a stop when the system isnot under load.

The prior art and a preferred embodiment of the invention will befurther explained below with reference to the drawing enclosed, inwhich:

FIG. 1 shows a passive hydraulic system with a retarding arrangementaccording to the prior art.

FIG. 2 shows a passive hydraulic system with a retarding arrangementaccording to the invention.

In FIG. 1, 1 denotes a primary hydraulic cylinder/piston unit arrangedin a lower tool half of a pressing tool for sheet metal forming. Theprimary hydraulic cylinder/piston unit 1 forms a passive system togetherwith a secondary hydraulic cylinder/piston unit 2 and a flow element 3connecting the cylinders. The passive system derives its driving forcefrom an external action on the piston rod of the primary hydrauliccylinder/piston unit 1. The main function of the system is to convert aforce in one direction at one point to a force in another direction atanother point. The flow element 3 is provided with a valve 4, which isdesigned to selectively permit a flow between the primary and thesecondary hydraulic cylinder/piston unit.

The retardation of the oil flow is achieved by means of the valve 4 anda gas-filled spring 5, which is arranged in the upper part of thepressing tool and designed to act in opposition to the piston rod of theprimary hydraulic cylinder/piston unit 1 when the tool is being closed.The valve 4 is kept closed when the tool is closing, for which reasonthe gas-filled spring 5 is compressed against the piston rod of theprimary hydraulic cylinder/piston unit 1, which remains in an extendedposition. When the tool is closed, the valve 4 is made to open at anarbitrary time, following which the piston rod of the gas-filled spring5 presses the piston rod of the primary hydraulic cylinder/piston unit 1in and oil flows from the latter to the secondary hydrauliccylinder/piston unit 2 so that its piston rod moves outwards. In orderfor this to function satisfactorily, the tool must be fully closed whenthe valve 4 is being opened.

FIG. 2 shows an arrangement according to the invention for shutting offor retarding of an oil flow in a hydraulic system. In this embodimentthe hydraulic system is a passive system of the same type as in FIG. 1.A primary hydraulic system/piston unit 7 is arranged in a lower toolhalf of a pressing tool for sheet metal forming and connected to aprimary flow connection of the retarding arrangement. A secondaryhydraulic cylinder/piston unit 10 is connected to a secondary flowconnection of the retarding arrangement and designed to perform anotheroperation in the same tool. A non-return valve 6 is designed to permit aflow in one direction from the primary flow connection to a pressurisedaccumulator 8, for example a piston accumulator. A first two way valve 9is designed to selectively permit a flow between the accumulator 8 andthe secondary flow connection. This can occur at any time after the toolis closed and also after it is opened again. A second two-way valve 11is arranged parallel with the non-return valve 6 and designed toselectively permit a return flow to the primary flow connection after acertain time or as a function of the position of the piston rod of thesecondary hydraulic cylinder/piston unit 10. The secondary hydrauliccylinder/piston unit 10 has a built-in return force which can beachieved by means of a gas-filled or coil spring.

When the halves of the sheet metal forming tool are closed, the pistonrod of the primary hydraulic cylinder/piston unit 7 is pressed into thecylinder. The oil flows from the latter in through the primary flowconnection. Since the second two-way valve 11 is normally closed at thistime, the oil flow passes the non-return valve 6 and continues to thepressurised accumulator 8. The oil is forced into the accumulator 8, inwhich there is a piston 12 which separates the oil frompressure-generating means. Gas-filled or coil springs, for example, mayserve as pressure-generating means. The piston 12 is pressed away by theoil which is pressurised by the piston 12. The pressure in theaccumulator 8 is equal to or greater than the working pressure in thehydraulic system. As long as the second two-way valve 11 or the firsttwo-way valve 9 respectively is kept closed, the oil flow through themcan be shut off or retarded in the accumulator 8.

When force is required from the secondary hydraulic cylinder/piston unit10, the first two-way valve 9 can be opened so as to permit a flowbetween the accumulator 8 and the secondary hydraulic cylinder/pistonunit 10. Its piston rod is made to move outwards as oil is forced in bythe piston 12 of the accumulator 8. The accumulator 8 also functions asoverload protection in case the piston rod is subjected to a required orunwanted stop. This is possible in that it is designed to accommodatethe oil which is present in the system when the piston rods of bothhydraulic cylinder/piston units 7, 10 are pressed into the cylinders. Insuch cases the accumulator 8 takes up the excess oil in the system sothat an undesirable pressure increase cannot occur. When the secondaryhydraulic cylinder/piston unit 10 has performed its stroke and theprimary hydraulic cylinder/piston unit 7 is relieved, the system can bemade to return to its basic position. Relief is often effected byopening the tool again immediately after closing it. This return strokecan be effected by the secondary hydraulic cylinder/piston unit 10,which is provided with a return force, forcing the oil back to theprimary hydraulic cylinder/piston unit 10. The return flow can occurafter the second two-way valve 11 has also been opened. The system isthereby again ready for a new forming operation.

The system according to the invention can also be adapted to a situationin which strokes by the secondary hydraulic cylinder/piston unit 10 arenot desirable. In this situation oil, as previously described, isdischarged from the primary hydraulic cylinder/piston unit through thenon-return valve 6 to the accumulator 8. Oil is then quite simplydischarged back to the relieved primary hydraulic cylinder/piston unit 7by opening of the second valve 11 after the tool has been opened. Thefirst two-way valve 9 is kept closed throughout the entire process. Inthis way a shutting-off of the secondary hydraulic cylinder/piston unit10 is achieved.

The piston 12 in the accumulator 8 can be designed to rest against astop when the system is not under load. This means that the pressure inthe unloaded system is zero in all parts outside the accumulator 8. Theaccumulator 8 can also consist of a spring-loaded piston rod in ahydraulic cylinder/piston unit.

Although the invention has been described in relation to sheet-metalforming equipment, it can be applied in any hydraulic system where thereis a need for retardation.

The invention is obviously not limited to the embodiment described aboveby way of an example, but lends its self to modifications within thescope of the idea of the invention defined in the claims below.

I claim:
 1. Arrangement in a pressing tool for sheet-metal forming inorder to shut off or retard an oil flow in a hydraulic system inco-ordination with the closing and opening movements of the said tool,characterised in that it comprises a non-return valve (6) designed, whenthe tool is being closed, to permit a flow in one direction from aprimary hydraulic cylinder/piston unit (7), which is thereby compressed,to a pressurised accumulator (8); a first valve (9) designed toselectively permit a flow between the accumulator (8) and a secondaryhydraulic cylinder/piston unit (10), which is designed as a consequenceof such a flow to move from a compressed basic position to an expandedposition; and parallel with the non-return valve (6) a second valve(11), designed so that when the primary hydraulic cylinder/piston unit(7) has been relieved by opening of the tool, it will selectively permita return flow to the primary hydraulic cylinder/piston unit (7) from theaccumulator (8) or, if the first valve (9) is also made to open, fromthe secondary hydraulic cylinder/piston unit (10), which is designed tobe then returned to the compressed basic position, and wherein theaccumulator (8) is of dimensions sufficient to accommodate the oil whichis in the system when both the hydraulic cylinder/piston units arecompressed.
 2. Arrangement according to claim 1, characterised in thatthe pressure in the accumulator (8) is always equal to or greater thanthe working pressure in the hydraulic system.
 3. Arrangement accordingto claim 2, characterised in that a piston (12) which separates the oilfrom pressure generating means is located in the accumulator (8). 4.Arrangement according to claim 3, characterised in that gas is used asthe pressure generating means.
 5. Arrangement according to claim 3,characterised in that a coil spring is used as pressure generatingmeans.
 6. Arrangement according to any of claims 1, characterised inthat the secondary hydraulic cylinder/piston unit (10) is designed to bereturned to the compressed basic position by means of a gas-filledspring.
 7. Arrangement according to any of claims 1, characterised inthat the secondary hydraulic cylinder/piston unit (10) is designed to bereturned to the compressed basic position by means of a coil spring. 8.Arrangement according to any of claims 1, characterised in that thesecondary hydraulic cylinder/piston unit (10) is designed to be returnedto the compressed basic position by means of a gas under pressure. 9.Arrangement according to claim 1, characterised in that the piston (12)in the accumulator (8) is designed to rest against a stop when thesystem is not under load.